1. Field of the Disclosure
Exemplary embodiments of the present disclosure relate to a heat conduction unit, a fixing device, and an electrophotographic or electrostatic image forming apparatus, such as a facsimile, printer, copier, or multifunction devices having at least two of the foregoing capabilities.
2. Description of the Background Art
As one type of image forming apparatus, electrophotographic image forming apparatuses are widely known. In an image formation process executed by an electrophotographic image forming apparatus, for example, a charger uniformly charges a surface of an image carrier (e.g., photoconductor drum); an optical writing unit emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium, such as a recording sheet, or indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device includes, e.g., a rotational fixing unit formed with a roller, a belt, or a combination of a roller and a belt. The fixing device sandwiches a recording medium at a fixing nip and applies heat and pressure to a toner image on the recording sheet to fix the toner image on the recording medium. Several types of fixing devices are conventionally known, including, for example, a belt-type fixing device.
FIG. 1 is a schematic view illustrating a conventional belt-type fixing device configuration. In FIG. 1, the belt-type fixing device includes a heating roller 102, a fixing roller 104, a fixing belt 101, and a pressing roller 109. The heating roller 102 includes a heater 103. The fixing roller 104 includes a rubber layer on its surface. The fixing belt 101 is stretched between the heating roller 102 and the fixing roller 104. The pressing roller 109 presses against the fixing roller 104 via the fixing belt 101 to form a fixing nip N. When a toner image is transferred onto a recording medium P, the recording medium P is conveyed to the fixing nip N between the fixing belt 101 and the pressing roller 109. When the recording medium P passes the fixing nip N, heat and pressure are applied to the toner image on the recording medium P to fix the toner image on the recording medium P.
FIG. 2 is a schematic view illustrating a conventional film-type fixing device configuration. As described in JP-H04-044075-A, typically, a ceramic heater 113 and a pressing roller 119 sandwiches a heat-resistant film 111 (equivalent to the fixing belt) to form the fixing nip N. A recording sheet is fed to the fixing nip N between the heat-resistant film 111 and the pressing roller 119. Then, the recording sheet is sandwiched by the heat-resistant film 111 and the pressing roller 119 to be conveyed together with the heat-resistant film 111. At this time, at the fixing nip N, heat of the ceramic heater 113 is applied to the recording medium with pressure via the heat-resistant film 111 to fix a toner image on the recording medium.
At the same time, however, belt-type fixing devices are not problem-free. For the belt-type fixing device, a large heat capacity of the fixing roller increases the time required for raising the temperature of the fixing roller to the requisite level for good image formation, resulting in an increased warm-up time.
To cope with such challenges, for example, JP-2007-334205-A proposes a fixing device that can shorten the warm-up time without increasing the heat capacity of the fixing belt. However, since the heat capacity of the typical pipe-shaped heat conductor is low, the heat conductor may be directly affected by the heat distribution of the heater. As a result, contact of the fixing belt and the heat conductor may change the temperature of the fixing belt.
In general, a uniform temperature distribution over the surface of the fixing belt is desirable. For the fixing device, the surface temperature distribution of the fixing belt may be affected by the heat distribution of the heater and the contact face of the heat conductor and the fixing belt, preventing uniform temperature distribution. Moreover, the fixing belt while rotating may be separated from the metal heat conductor at a certain position, such that heat from the metal heat conductor is not transferred to the fixing belt. Consequently, the metal heat conductor may be overheated, resulting in an increased rotation torque of the fixing belt. Additionally, the fixing device transfers heat of the resistant heat generator to an opposing member, resulting in a limitation in shortening of the warm-up time and/or the first print time.
To cope with such challenges, JP-2008-216928-A proposes a fixing device including an endless-shaped fixing belt, a pressing roller pressed against the fixing belt to form a nip through which the recording medium is conveyed, and a resistant heat generator provided inside a loop formed by the fixing belt to heat the fixing belt. The resistant heat generator is provided slightly away from the inner circumferential face of the fixing belt so as not to press against the inner circumferential face of the fixing belt, and the fixing belt is entirely heated by radiation heat radiated from the resistant heat generator.
However, for the fixing device, since the fixing belt is positioned adjacent to the resistant heat generator to suppress a reduction in heating efficiency, a portion of the flexible fixing belt while rotating may come into contact with the resistant heat generator. As a result, heat from the resistant heat generator is transferred to the contact portion of the fixing belt. Thus, the fixing belt is heated in a non-uniform manner, resulting in non-uniform temperature distribution over the surface of the fixing belt.
Further, there is another consideration. It is generally presupposed that different types of recording media pass through the fixing device, or, put differently, that the apparatus incorporating the fixing device can accommodate recording media of multiple different sizes. For example, assume that a relatively small recording medium smaller than an axial width of a heat generation area of a heater for heating the fixing member passes through the fixing device. In this state, since heat from an area of the fixing member over which the sheet of recording media does not pass (typically the axial end portions of the fixing member) is not absorbed by the recording media, these end portions may get overheated (i.e., the temperature may increase excessively), degrading the fixing member and reducing product life.
Hence, JP-2008-310051-A proposes a fixing device in which multiple heat sources (e.g., halogen heaters, planar heat generators, or electromagnetic induction heaters) having different heating distributions in the width direction of the recording media are provided as heaters and power is supplied only to at least one of the heat sources compatible with the sheet pass width of the recording medium to prevent temperature increase in the end portions of the fixing member.
Although successful for its intended purpose, the fixing device of JP-2008-310051-A has limitations on the sizes of the recording media that it can accommodate because the width of the heat generation area can be adjusted only by changing the number of heat sources. Further, although the fixing device described in JP-2008-216928-A has a plurality of resistant heat generators arranged in an axial direction of the fixing belt and the resistant heat generators are controlled independently, so that the heating distribution in the axial direction of the fixing belt can be adjusted, nevertheless the fixing device also has a limitation in flexible response to different sizes of recording media.